You don't blow the brake duct opening. If you look carefully, the teams all run lots of airfoils attached to the outside of the brake ducts. There's a small area that is unregulated, and they cram as many downforce generating elements in there as possible.
It probably does not generate obscene downforce levels, but it certainly generates a significant amount. More importantly, this downforce is unsprung. It acts directly on the wheel, as opposed to having to go through the chassis and springs. The main advantage of this is it reduces the pitch sensitivity, as the only movement will be from tyre flex. Done right, you will not heat the brakes appreciably by blowing these elements.